1. Field of the Invention
The present invention relates to a magnetoresistive device that incorporates a magnetoresistive element and a method of manufacturing such a magnetoresistive device, and to a thin-film magnetic head that incorporates a magnetoresistive element and a method of manufacturing such a thin-film magnetic head.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading.
MR elements include: an AMR element that utilizes the anisotropic magnetoresistive effect; a GMR element that utilizes the giant magnetoresistive effect; and a TMR element that utilizes the tunnel magnetoresistive effect.
Read heads that exhibit a high sensitivity and a high output are required. Read heads that meet these requirements are GMR heads incorporating spin-valve GMR elements. Such GMR heads have been mass-produced.
Another characteristic required for read heads is a small Barkhausen noise. Barkhausen noise results from transition of a domain wall of a magnetic domain of an MR element. If Barkhausen noise occurs, an abrupt variation in output results, which induces a reduction in signal-to-noise (S/N) ratio and an increase in error rate.
To reduce Barkhausen noise, a bias magnetic field (that may be hereinafter called a longitudinal bias field) is applied to the MR element along the longitudinal direction. To apply a longitudinal bias field to the MR element, bias field applying layers may be provided on both sides of the MR element, for example. Each of the bias field applying layers is made of a hard magnetic layer or a laminate of a ferromagnetic layer and an antiferromagnetic layer, for example.
In a read head in which bias field applying layers are provided on both sides of the MR element, two electrode layers for feeding a current used for signal detection (that may be hereinafter called a sense current) to the MR element are located to touch the bias field applying layers.
As disclosed in Published Unexamined Japanese Patent Application Heisei 11-31313 (1999), it is known that, when the bias field applying layers are located on both sides of the MR element, regions that may be hereinafter called dead regions are created near ends of the MR element that are adjacent to the bias field applying layers. In these regions the magnetic field produced from the bias field applying layers fixes the direction of magnetization, and sensing of a signal magnetic field is thereby prevented.
Consequently, if the electrode layers are located so as not to overlap the MR element, a sense current passes through the dead regions. The output of the read head is thereby reduced.
To solve this problem, the electrode layers are located to overlap the MR element, as disclosed in Published Unexamined Japanese Patent Application Heisei 8-45037 (1996), Published Unexamined Japanese Patent Application Heisei 9-282618 (1997), Published Unexamined Japanese Patent Application Heisei 11-31313 (1999), and Published Unexamined Japanese Patent Application 2000-76629, for example.
Attention is now focused on the length of the region of one of the electrode layers that is laid over the MR element, that is, the distance between an end of the one of the electrode layers and one of the ends of the MR element that corresponds to this end of the one of the electrode layers. This length or distance is hereinafter called an overlap amount. No particular range of overlap amount is disclosed in Published Unexamined Japanese Patent Application Heisei 8-45037. The range of overlap amount disclosed in Published Unexamined Japanese Patent Application Heisei 9-282618 is 0.25 to 2 μm. The range of overlap amount disclosed in Published Unexamined Japanese Patent Application Heisei 11-31313 is 0.15 to 0.5 μm. The range of overlap amount disclosed in Published Unexamined Japanese Patent Application 2000-76629 is 0.15 to 5 μm.
It is possible to reduce Barkhausen noise while a reduction in output of the read head is prevented, if the read head has a structure that the bias field applying layers are located on both sides of the MR element, and the electrode layers overlap the MR element, as described above. Such a structure is hereinafter called an overlapping electrode layer structure.
In general, a spin-valve GMR element incorporates: a nonmagnetic layer having two surfaces that face toward opposite directions; a soft magnetic layer that is located adjacent to one of the surfaces of the nonmagnetic layers; a ferromagnetic layer that is located adjacent to the other of the surfaces of the nonmagnetic layers; and an antiferromagnetic layer that is located adjacent to one of surfaces of the ferromagnetic layer that is farther from the nonmagnetic layer. The soft magnetic layer is a layer in which the direction of magnetization varies in response to a signal field, and is called a free layer. The ferromagnetic layer is a layer in which the direction of magnetization is fixed by the field produced from the antiferromagnetic layer, and is called a pinned layer.
The inventors of the present invention found out that, in the read head of the overlapping electrode layer structure that incorporates the spin-valve GMR element having the above-mentioned structure, the state of magnetization in the free layer is uneven, and it is thereby impossible to fully reduce Barkhausen noise. The reason for the uneven magnetization in the free layer will be described in detail in the description of preferred embodiments of the invention.
The inventors of the invention found out that, in the read head of the overlapping electrode layer structure, there is a difference between the space between the two electrodes, that is, the optical magnetic read track width, and the effective magnetic read track width. Furthermore, in the ranges of overlap amount disclosed in the above-mentioned publications, there is a great difference between the optical magnetic read track width and the effective magnetic read track width, and there is a great variation in effective magnetic read track width, which is a problem that affects the properties of the read head and the yield.
According to a technique disclosed in Published Unexamined Japanese Patent Application 2000-187813, the ratio L1/L2 is 0 to 10% wherein L2 is the width of the sensing portion of the spin-valve film and L1 is the length of the permanent magnet film and the electrode film that overlap the sensing portion. This technique is aimed at preventing noise caused by the permanent magnet film overlapping the spin-valve film. Although the structure in which only the permanent magnet film overlaps the sensing portion and the structure in which the permanent magnet film and the electrode film overlap the sensing portion are disclosed in this publication, the structure in which only the electrode film overlaps the sensing portion while the permanent magnet film does not overlap the sensing portion is not disclosed.